netdev - Re: [PATCH net-next-2.6 v3] can: Topcliff: PCH

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Date:	Wed, 24 Nov 2010 13:34:24 +0100
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Tomoya MORINAGA <tomoya-linux@....okisemi.com>
CC:	andrew.chih.howe.khor@...el.com, socketcan-core@...ts.berlios.de,
	Samuel Ortiz <sameo@...ux.intel.com>, margie.foster@...el.com,
	netdev@...r.kernel.org, Christian Pellegrin <chripell@...e.org>,
	linux-kernel@...r.kernel.org, yong.y.wang@...el.com,
	kok.howg.ewe@...el.com, joel.clark@...el.com,
	"David S. Miller" <davem@...emloft.net>,
	Wolfgang Grandegger <wg@...ndegger.com>, qi.wang@...el.com
Subject: Re: [PATCH net-next-2.6 v3] can: Topcliff: PCH_CAN driver: Add Flow
 control,

On 11/24/2010 01:09 AM, Tomoya MORINAGA wrote:
> On Monday, November 22, 2010 5:27 PM, Marc Kleine-Budde wrote:
>>>>>> Still we have the busy waiting in the TX path. Maybe you can move the
>>>>>> waiting before accessing the if[1] and remove the busy waiting here.
>>>>> I can't understand your saying.
>>>>> For transmitting data, calling pch_can_rw_msg_obj is mandatory.
>>>> Yes, but the busy wait is not needed. It should be enough to do the
>>>> busy-waiting _before_ accessing the if[1].
>>>
>>> Do you mean we should create other pch_can_rw_msg_obj which doesn't have busy wait ?
>> ACK, and this non busy waiting is use in the TX path. But you add a busy
>> wait only function before accessing the if[1] in the TX path.
> 
> The "busy waiting" of pch_can_rw_msg_obj is for next processing accesses to Message object.
> If deleting this busy waiting, next processing can access to Message object, regardless previous transfer doesn't
> complete yet.
> Thus, I think, the "busy waiting" is necessary.

Yes, it's necessary, but not where it is done currently.
Let me outline how I think the TX path should look like:

pch_xmit() {
	take_care_about_flow_control();
	prepare_can_frame_to_be_copied_to_tx_if();

	/* most likely we don't have to wait here */
	wait_until_tx_if_is_ready();

	copy_can_frame_to_tx_if();

	/* trigger tx in hardware */
	send_tx_if_but_dont_do_busywait();

	/* tx_if is busy now, but before we access it, we'll check tx_if is ready */
}

cheers, Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
Vertretung West/Dortmund          | Fax:   +49-5121-206917-5555 |
Amtsgericht Hildesheim, HRA 2686  | http://www.pengutronix.de   |


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